Chapter 13: The Genetic Code and Transcription Flashcards
What polymerase is responsible for transcription?
RNA polymerase
True or false: a primer is needed for both DNA replication and transcription.
False. A primer isn’t needed for transcription because it uses RNA polymerase, not DNA polymerase.
Does RNA polymerase use DNA or RNA nucleotides for transcription?
RNA nucleotides
What four things are required for transcription?
- All four ribonucleotide 5’ triphosphates
- A DNA template strand
- 5’ to 3’ synthesis
- Control sequences
RNA polymerase is a _____, which is composed of multiple subunits.
holoenzyme
What does the sigma subunit of the RNA polymerase holoenzyme do?
It recognizes the promotor sequences on the DNA.
This control sequence is present in all genes and does not get copied, but signals the polymerase to start at that site.
Promoter
Prokaryotes only have _____ holoenzymes, while eukaryotes have _____.
five; thirty
How does polymerase know which DNA strand to copy?
Only one strand has the promoter sequence.
Where is the promoter sequence located?
Towards the 5’ end.
Does the sigma subunit stay with the holoenzyme as it elongates the chain?
No, it falls off after the holoenzyme has found the promoter and starts working.
These are sequences of DNA that are similar in different genes of the same organism or in related organisms.
Consensus sequences
What are the two consensus sequences in prokaryotes?
-10 TATAAT and -35 TTGACA
What is the Pribnow box, and where is it located?
TATAAT, located at -10.
Where is the TTGACA consensus sequence located?
-35
What are the two consensus sequences in eukaryotes?
-35 TATAAATA and -80 GGCCAATCT
Where is the TATAAATA sequence located?
-35
Where is the GGCCAATCT sequence located?
-80
What’s the CAAT box?
GGCCAATCT
Why are consensus sequences so heavy on A’s and T’s?
They have only two hydrogen bonds, making them easier to open.
How is efficiency affected by protein demand?
Proteins in high demand must be transcribed very efficiently, while low demand ones can be off by a nucleotide or two because they aren’t needed as much.
What are the two types of promoters?
Focused and dispersed
This type of promoter appears only once in the gene, meaning there’s only one start site for polymerase so it always starts on the same spot.
Focused
This type of promoter appears multiple times, offering several start sites.
Dispersed
This type of promoter appears in both prokaryotes and eukaryotes and is highly conserved.
Focused
This type of promoter appears only in eukaryotes.
Dispersed
What does a transcription termination site in prokaryotes usually look like?
A string of A’s preceded by a palindrome.
How does polymerase know when to stop transcribing in prokaryotes?
The palindrome at the end of the sequence makes a little hairpin loop by base pairing with itself, and when polymerase hits it, it knows to copy it and then stop.
What happens in prokaryotes after the mRNA comes out of the polymerase?
Since there’s no nucleus, the ribosomes are ready and waiting for the mRNA to wrap around them to be translated into proteins.
What happens in eukaryotes after the mRNA comes out of the polymerase?
tRNAs are made in the nucleus, and then they go out to the ribosomes to be made into protein.
How can you tell a prokaryotic mRNA from a eukaryotic mRNA just by looking at it?
Prokaryotic ones will have ribosomes, while eukaryotic ones will not.
Does polymerase have to finish transcribing the RNA before another one can start?
No, multiple polymerases can be on at once. They jump on wherever there’s promoter.
What happens to the promoter once it falls off?
It’s ready for the next.
How can you tell between a really efficient gene and a less efficient gene by looking at transcription?
More efficient genes will have a lot of mRNAs coming off of them, because they are being transcribed quickly by multiple polymerases. Less efficient ones will only have a few.
In _____, transcription and translation occur in the same place, while in _____, it occurs in two different locations (the nucleus and the ribosome, respectively).
prokaryotes; eukaryotes
How many polymerases are there in eukaryotes?
Three
This polymerase makes rRNA in the nucleolus.
Polymerase I
This polymerase makes mRNA and snRNA in the nucleoplasm.
Polymerase II
This polymerase makes 5S rRNA and tRNA in the nucleoplasm.
Polymerase III
This is a sequence at the 5’ end of an mRNA, right before translation initiation begins, that helps facilitate translation.
Leader sequence
This is a sequence on an mRNA on the 3’ end where translation termination begins.
Trailer
True or false: In eukaryotes, RNA made by polymerase is not yet ready to be made into protein directly after transcription.
True
What three steps must be completed before RNA is ready for translation in eukaryotes?
- It must be transcribed into pre-mRNA by polymerase, making a copy of the DNA strand.
- A cap and tail must be added.
- The mRNA’s introns must be removed and its exons spliced together.
How stable are mRNAs, and why?
They’re very delicate and have a short lifetime, because as long as they are around, proteins will be translated from them. If they were around forever, we’d make that protein forever.
Proteins that are factors that help with transcription initiation are called _____.
Transcription factors
This transcription factor binds directly to the TATA box, recruiting other transcription factors to form the pre-initiation complex.
TFIID
This is a cap placed on new RNAs during DNA processing as soon as it comes off the polymerase.
7-methylguanine
This has three phosphates attached to it, but the phosphodiester bond is 5’ to 5’.
7-methylguanine
Does the poly-A tail get put on before or after the cap?
After.
True or false: Polymerase stops when it gets to the polyadenylation signal, and the tail is placed on the end of the new RNA.
False. Polymerase keeps going through that signal, the cleavage signal, and the stop signal.
This enzyme sees the poly-A signal get transcribed and cuts the RNA directly after.
Clipping enzyme complex, or endonuclease
True or false: endonuclease is a separate unit from the polymerase.
False. It is attached to it.
What happens after endonuclease cuts the RNA?
A string of A’s gets put on the end.
What happens to the polymerase after endonuclease clips the new RNA strand off?
It keeps going down the DNA strand, making an RNA strand.
What stops polymerase after the RNA is clipped?
Exonuclease nibbling back the RNA and hitting polymerase.
This enzyme jumps on the strand that polymerase continues to make after the RNA has been clipped and polyadenylated, then nibbles the extra RNA. When it reaches polymerase, transcription stops.
Exonuclease
_____ is intron removal from pre-RNA to make mRNA.
Splicing
True or false: intron removal occurs only in prokaryotes.
False. Prokaryotes do not have introns and exons; this only occurs in eukaryotes.
How are introns removed?
They form loops with the help of proteins, then get cut off by an enzyme. The exons are then sealed together by ligase.
True or false: introns are much larger than exons.
True.
Why are introns so much larger than genes if they’re just going to be removed?
They allow a single gene to make more than one protein.
This stands for small nuclear ribonuclearprotein particles.
snRNP
This is composed of multiple snRNPs assembled at splicing junctions.
Spliceosome
What is the role of the spliceosome?
It pulls the ends of the intron together, cuts it, and splices the exons.
True or false: Intron removal can be off by a nucleotide or two.
False.
Why must intron removal be incredibly precise?
If it’s off by even one nucleotide, you’re getting into the exons and that changes the sequence, introducing a mutation.
